Charlemagne's army is fighting the Muslims in Spain. The last city standing is Saragossa, held by the Muslim king Marsilla. Terrified of the might of Charlemagne's army of Franks, Marsilla sends out messengers to Charlemagne, promising treasure and Marsilla's conversion to Christianity if the Franks will go back to France. Charlemagne and his men are tired of fighting and decide to accept this peace offer. They need now to select a messenger to go back to Marsilla's court. The bold warrior Roland nominates his stepfather Ganelon. Ganelon is enraged; he fears that he'll die in the hands of the bloodthirsty pagans and suspects that this is just Roland's intent. He has long hated and envied his stepson, and, riding back to Saragossa with the Saracen messengers, he finds an opportunity for revenge. He tells the Saracens how they could ambush the rear guard of Charlemagne's army, which will surely be led by Roland as the Franks pick their way back to Spain through the mountain passes, and helps the Saracens plan their attack.
Just as the traitor Ganelon predicted, Roland gallantly volunteers to lead the rear guard. The wise and moderate Olivier and the fierce archbishop Turpin are among the men Roland picks to join him. Pagans ambush them at Roncesvals, according to plan; the Christians are overwhelmed by their sheer numbers. Seeing how badly outnumbered they are, Olivier asks Roland to blow on his oliphant, his horn made out of an elephant tusk, to call for help from the main body of the Frankish army. Roland proudly refuses to do so, claiming that they need no help, that the rear guard can easily take on the pagan hordes. While the Franks fight magnificently, there's no way they can continue to hold off against the Saracens, and the battle begins to turn clearly against them. Almost all his men are dead and Roland knows that it's now too late for Charlemagne and his troops to save them, but he blows his oliphant anyway, so that the emperor can see what happened to his men and avenge them. Roland blows so hard that his temples burst. He dies a glorious martyr's death, and saints take his soul straight to Paradise.
When Charlemagne and his men reach the battlefield, they find only dead bodies. The pagans have fled, but the Franks pursue them, chasing them into the river Ebro, where they all drown. Meanwhile, the powerful emir of Babylon, Baligant, has arrived in Spain to help his vassal Marsilla fend off the Frankish threat. Baligant and his enormous Muslim army ride after Charlemagne and his Christian army, meeting them on the battlefield at Roncesvals, where the Christians are burying and mourning their dead. Both sides fight valiantly. But when Charlemagne kills Baligant, all the pagan army scatter and flee. Now Saragossa has no defenders left; the Franks take the city. With Marsilla's wife Bramimonde, Charlemagne and his men ride back to Aix, their capital in sweet France.
The Franks discovered Ganelon's betrayal some time ago and keep him in chains until it is time for his trial. Ganelon argues that his action was legitimate revenge, openly proclaimed, not treason. While the council of barons, which Charlemagne gathered to decide the traitor's fate is initially swayed by this claim, one man, Thierry, argues that, because Roland was serving Charlemagne when Ganelon delivered his revenge on him, Ganelon's action constitutes a betrayal of the emperor. Ganelon's friend Pinabel challenges Thierry to trial by combat; the two will fight a duel to see who's right. By divine intervention, Thierry, the weaker man, wins, killing Pinabel. The Franks are convinced by this of Ganelon's villainy and sentence him to a most painful death. The traitor is torn limb from limb by galloping horses and thirty of his relatives are hung for good measure.